Integrated phase-shift power control transmitter for use with implantable device and method for use of the same

ABSTRACT

Systems and methods for efficiently transmitting power using a high frequency (e.g., RF) telemetry transmitter are provided. The telemetry transmitter may include a fixed clock source (which may provide a fixed clock signal), telemetry phase shift circuitry (which may include switching circuitry and phase shifting circuitry), and a push-pull network. The telemetry phase shift circuitry generates a phase shifted clock signal that is phase shifted with respect to the fixed clock signal. The fixed and phase shifted clock signals may drive the switching circuitry to produce a high frequency signal that is passed through the push-pull network. The power or magnitude of the high frequency signal is based on the phase delay between the fixed clock signal and the phase shifted clock signal.

BACKGROUND OF THE INVENTION

The present invention relates to methods and systems for controlling thepower level of high frequency signals, and more particularly to RFtelemetry transmitters for efficiently communicating with and poweringan implanted stimulator device (e.g., an implanted cochlear stimulator(ICS)).

Cochlear implant technology is well known and has been successfully usedto enable individuals to hear, whereas other hearing assist devices,such as hearing aids and head phone amplifiers, have failed. Generally,cochlear implant systems include an external unit and an implanteddevice. The external unit usually includes a power source (e.g., abattery), where the implanted device may not. The implanted device mayreceive power from the external unit by way of an inductive or radiofrequency (RF) link. To transfer power from the external unit to theimplanted device, the external unit and implanted device may eachinclude a coil. Although these coils are not directly connected, a highfrequency carrier signal, which is applied to the external device coil,is coupled to the implanted device coil. This coupling is akin to theflux coupling seen in transformers. That is, even though the primary andsecondary windings are not directly coupled to each other, an AC signalapplied to the primary winding is also applied to the secondary windingby virtue of the flux coupling. In an ICS system, the carrier signal isreceived by the implanted coupling and then rectified into a DC signalfor powering the implanted device.

Control and/or data signals may be transmitted to the implanted deviceby applying a predetermined modulation signal to the carrier signal. Forexample, acoustic signals received and processed by the external devicemay be converted into electrical signals (e.g., a digital pulse stream),which may provide a basis for the modulation applied to the carriersignal.

Cochlear implant systems, like many other electronic systems, areconstantly subject to ever stringent design criteria such as smallersize requirements, greater power efficiency, and lower costs. One way toaddress each of the foregoing design criteria, and others not mentioned,is to increase the efficiency of power conversion and transfer from theexternal unit to the implanted unit. Traditional power conversion andtransfer techniques, although are able to provide power to the implanteddevice, do not completely meet the stringent criteria. As a result, theexternal unit of the cochlear implant systems may require bulkyhousings, large power requirements, and frequent replacement or chargingof batteries.

One example of a known power conversion technique uses a class D, E/F,G, H, or S transmitter in combination with a voltage regulator (e.g., aswitching regulator). The voltage regulator controls the transmittersupply voltage, which control is responsible for adjusting the poweroutput of the transmitter. A drawback with this technique is that thevoltage regulator requires additional circuitry such as controlcircuitry and discrete components such as inductors and capacitors tooperate. These additional components add costs, consume additionalpower, and occupy extra space.

Another power conversion technique eliminates the need to use a voltageregulator to control the transmitter power output by using a pulse widthmodulation (PWM) technique (or duty cycle control) to adjust themagnitude of the carrier signal. One drawback of using PWM to controlthe transmitter power output is that both even and odd higher orderharmonics may be imposed on the carrier signal. As is known in the art,harmonics represent unwanted components of a signal (e.g., a carriersignal) that are typically produced in high frequency applications. Theproduction of both even and odd harmonics places a substantial burden onfiltering circuitry because if the higher order harmonics currents arenot suppressed, these harmonic currents can decrease the powerefficiency of the transmitter circuitry. The production of undesirablesecond harmonics increases the steepness of the suppression filtercircuits, which generally increases the order of the filter and theinherent losses in such filters. The radiation of the harmoniccomponents may generate EMI (e.g., Electro-Magnetic Interference), whichis often prohibited or regulated. Moreover, another drawback is that thecontrol of the duty cycle may be difficult as it approaches zero, thuspotentially preventing accurate control of power across the entireavailable range of power that can be transmitted on the carrier signal.

Because many cochlear implant devices are implemented in relativelysmall behind-the-ear units, space and power are at a premium.Furthermore, as cochlear implant devices advance, other components suchas digital processing circuitry may require increased levels of powerand space. Thus, there is a need for a high frequency transmittercircuit that is both compact and efficient.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providingan efficient RF telemetry transmitter system for transmitting power anddata to an implant device. The transmitter system uses a phase delay(e.g., time delay or phase shift) technique to control the power levelof the signal being transmitted to the implant device. Such a techniqueadvantageously eliminates the need of an additional voltage regulator(e.g., a switching voltage regulator) and its associated discretecomponents to control the voltage of the transmitted RF signal. Inaddition, this technique enables the transmitter to accurately controlthe magnitude of the RF signal to assure optimum power transfer to theimplant device. Thus, the amount of power provided to the implant devicecan be set to a level required by the implant device. That is, anoversupply (which may result in wasted power) and an undersupply (whichmay result in improper implant device operation) of power to the implantdevice is avoided.

The transmitter circuitry according to the invention may include acarrier frequency clock source, phase shifting circuitry, switchingcircuitry (which may include a first and second amplifier), and anetwork (which may include a transformer). The transmitter circuitry,particularly the switching circuitry and the network, may be setup in apush-pull configuration. This configuration enables digital signals(e.g., a predetermined frequency clock signal and a phase delayed clocksignal) to control the operation of the switching circuitry, which inturn causes signals to be applied to the network. The differentialsignal measured across a winding (e.g., a primary winding) of thetransformer, contains the desired fundamental frequency component, aswell as unwanted harmonic frequency components. The network suppress theharmonic components and passes the desired fundamental frequency asmodulated in amplitude by a phase-shift summation.

In one embodiment, the switching circuitry may include a first amplifierthat is responsive to a predetermined frequency clock signal to providea first switch signal to the network (e.g., to a first node of thetransformer). The switching circuitry may include a second amplifierthat is responsive to a phase shifted clock signal to provide a secondswitch signal to the network (e.g., to a second node of thetransformer). The phase shifted clock signal may be generated by thephase shifting circuitry. The phase shifted clock signal has the samefrequency as the predetermined frequency clock signal, but may beshifted out of phase with respect to the predetermined frequency clocksignal. For example, the phase difference between the leading edge ofthe phase shifted clock signal and the leading edge of the predeterminedfrequency clock signal circuitry may range from −180° to 0° to 180°, orin radian values −Π to 0 to Π.

During operation, a differential voltage exists across the transformerwhen the first and second switch signals are out of phase. The magnitudeof the phase difference may determine the average amount of power ormagnitude of power transmitted to, for example, an implant device. Forexample, if the phase delay is 0°, the power of the RF signaltransmitted through the network may be negligible. However, as the phasedelay approaches 180° or −180°, the power of the RF signal beingtransmitted increases, with maximum power being obtained at a 180° or−180° phase shift.

The present invention provides accurate power control over the fullrange (e.g., from the lowest possible order of magnitude to the highestpossible order of magnitude) of potential power by adjusting the phasedelay. Such adjustment of the phase delay can be done by providing aDELAY CONTROL signal to the phase shifting circuitry. For example, themagnitude (e.g., voltage level) of the DELAY CONTROL signal maydetermine the phase shift. The DELAY CONTROL signal may be provided bycontrol circuitry, feedback control circuitry, or other suitablecircuitry capable of providing a DELAY CONTROL signal. Moreover, suchcircuitry may be responsive to the power level being provided to, forexample, the implant device, and may make adjustments to the DELAYCONTROL signal to ensure that an optimum power level is provided.

In one embodiment of the present invention, the network may be adouble-tuned network that functions as a narrow band filter. The filterfunction may prevent higher order harmonics, which may be produced bythe driving action of the switching circuitry, from being passeddownstream of the network. In another aspect of the present invention,the switching circuitry is balanced, resulting in the production of oddharmonics, but not even harmonics. This is advantageous because itreduces the suppression filter complexity and losses as described above.

The transmitter circuitry according to the present invention maytransmit data to, for example, an implant device. Data may be embeddedinto the predetermined frequency clock signal using a predeterminedmodulation scheme (e.g., ON/OFF modulation). Thus, the RF signal mayprovide both power and data to the implant device.

It is thus an object of the present invention to provide a compact, lowpower, highly efficient, RF telemetry transmitter circuit that may beused to transfer RF power from a limited power source (e.g., a smallbattery) through a barrier, such as the skin, to a device on the otherside of the barrier (e.g., an implant device).

It is a further object of the invention to provide a compact, low power,highly efficient, RF telemetry transmitter for use within abehind-the-ear (BTE) unit of a cochlear implant system.

It is an additional object of the invention to provide a low power,highly efficient RF telemetry transmitter circuit wherein the drivelevel or energy content of a fixed frequency RF signal may be controlledthrough phase shifting of a predetermined frequency clock signal.

It is further still an object of the invention to provide a transmittercircuit that accurately provides power at practically any level rangingfrom a minimum power level to a maximum power level.

It is yet another object of the invention to provide a transmittercircuit that does not impose both even and odd harmonics on the carriersignal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts throughout, and in which:

FIG. 1A shows a block diagram of a prior art ICS system;

FIG. 1B shows a block diagram of a prior art transmitter modulator usedin the external speech processor (SP) of the ICS system;

FIG. 2 shows several main components of a BTE cochlear stimulationsystem in which transmitter circuitry according to the principles of thepresent invention may be used;

FIG. 3 shows a block diagram of transmitter circuitry and othercomponents that may be housed within the BTE housing of a BTE cochlearstimulation system according to the principles of the present invention;

FIG. 4 shows a schematic of transmitter circuitry according to theprinciples of the present invention;

FIG. 4A shows an illustrative, but more detailed, schematic of a portionof the transmitter circuit of FIG. 4 in accordance with the principlesof the present invention;

FIG. 5 shows several timing waveforms of that may be generated in thecircuitry of FIG. 4 in accordance with the principles of the presentinvention;

FIG. 6 shows several timing waveforms illustrating phase shifting inaccordance with the principles of the present invention;

FIG. 7 shows a graph of a signal in which the magnitude varies as afunction of phase delay in accordance with the principles of the presentinvention; and

FIG. 8 illustrates one manner in which the carrier signal may bemodulated with digital data in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a representative block diagram of an ICS system 12 of thetype commonly found in the prior art. A thorough description of acochlear stimulation system of the type shown in FIG. 1A may be found,for example, in U.S. Pat. No. 5,776,172, which is incorporated herein byreference in its entirety. As seen in FIG. 1A, the ICS system 12includes an implanted portion, including an ICS 14 and an electrodearray 16, and an external portion, comprising a speech processor (SP) 20connected to a headpiece 24 via a long cable 22. The implanted portionis separated from the external portion by a barrier such as, forexample, a layer of skin 18.

In operation, the prior art ICS system 12 of FIG. 1A transmits power anddata through the skin barrier 18 to the implantable portion using atechnique as illustrated generally in the functional block diagram ofFIG. 1B. The ICS system 12 uses the received data and power to providestimulation pulses to select electrode pairs located along the electrodearray 16, as is known in the art. The SP 20, as seen in FIG. 1B,includes an oscillator 26 that generates a carrier signal which isapplied to a power amplifier 27. The power amplifier is modulated withdata through, for example, an ON/OFF modulation switch 29, and theresulting modulated amplified signal is sent to the headpiece 24 overthe long cable 22, from which location it is transmitted to theimplantable part through an antenna coil 28 housed within the headpiece24.

Included within the SP 20 is a large replaceable battery 23, or otherlarge power source. Such a large power source 23 is needed because thetransmission scheme shown in FIG. 1B is not very efficient. That is, thepower amplifier 27 is designed to amplify a sinusoidal signal, and as isreadily known in the art, the amplification of sinusoidal signals isgenerally not an efficient process. This is because in order to amplifythe sinusoidal signal without distortion, the various amplificationstages, typically implemented using transistors, must operate in theirlinear (non-saturated) region. This means that significant amounts ofpower may be consumed, or lost, in the amplification stages as theamplification of the sinusoidal signal is carried out. While certainamplifier configurations may be selected in an attempt to make thesinusoidal amplification more efficient (e.g., the power amplifier 27may be a Class C amplifier) such amplifier configurations are still notall that efficient (a Class-C amplifier typically operates at about 55%collector efficiency) and require more components (and thus more space).Fortunately, for speech processor designs of the prior art, theefficiency and size of the SP 20 were not major design concerns becausethe SP is simply carried by the patient (thereby allowing it to berelatively large), and a large replaceable battery 23 housed within theSP 20 could simply be replaced, when needed. (Here, the term“sinusoidal” is used to refer to a sine wave signal having little or nodistortion.)

FIG. 2 shows the main components of a behind-the-ear (BTE) cochlearstimulation system 30 of the type that may be used in accordance withthe principles of the present invention. The implanted portion of thesystem 30 may include ICS system 14 and electrode array 16. The externalportion of the system 30 may include a BTE unit 32 coupled to aheadpiece 34 via a short cable 33. A skin barrier 18 separates theexternal portion from the implanted portion. The BTE unit 32 houses aspeech processor, a power source, the telemetry transmitter circuit ofthe present invention, as well as other standard components and speechprocessing circuits used within an ICS system.

Headpiece 34 houses an antenna coil, and may also house (in someembodiments) a microphone. In other embodiments, the microphone may behoused within or on BTE unit 32. BTE unit 32 is designed to be wornbehind the ear of its user, and the headpiece 34 is connected theretovia the short (i.e., less than two inches or so) cable 33.

Because BTE unit 32 shown in FIG. 2 is physically much smaller than isthe SP unit 20 used with the prior ICS system 12 (FIG. 1A), and furtherbecause all of the circuits used within the BTE system 30 must performsubstantially the same functions as are performed by equivalent circuitswithin the prior ICS system 12, it is necessary that such circuitsperform their respective functions much more efficiently (i.e., byconsuming less power than their prior art counterparts) because thepower source used within the BTE system 30 cannot be as large, and hencecannot generally have the same capacity as, the power source used withinthe prior SP unit 20. While improved power sources (batteries) may beused in the BTE system 20 that provide a higher energy density, andhence more power in a smaller space or volume than has heretofore beenachievable, it is still necessary that the circuits of the BTE system bedesigned with compactness and maximum efficiency in mind.

To that end, the present invention provides a highly efficient, compact,telemetry transmitter circuit for use in transmitting a high frequencycarrier signal across a barrier to a remote receiver (e.g., to animplanted receiver). Such an efficient, compact telemetry transmittercircuit, and related components, is illustrated in the block diagramshown in FIG. 3.

The high frequency carrier signal may be the signal that is transmittedto implanted ICS 14 across skin barrier 18. As will be referred toherein, the magnitude or amplitude of this high frequency carrier signalmay indicate the quantity of power being transmitted to the implantedICS 14. Moreover, as will also be described herein, the high frequencycarrier signal may also include data. Furthermore, the high frequencycarrier signal may be referred to herein as an RF signal, a highfrequency signal, or a sinusoidal signal.

As seen in FIG. 3, the transmitter circuit may include an oscillatorcircuit (OSC) 42 that generates a primary clock signal having afrequency F1. For example, the clock signal frequency F1 may be 49 MHz.The OSC 42 may also generate other clock signals such as a second clocksignal Fn, having a frequency that is the same as or derived from thefrequency F1 of the primary clock signal. For example, the second clocksignal may have a frequency of 24.5 MHz, or ½ that of the primary clocksignal.

It will be understood by those skilled in the art that the presentinvention may be use varying frequencies and is not limited to usingjust one particular frequency. For example, the transmitter circuit mayinclude clock generation circuitry that provides a clock signal thatvaries within a predetermined range of frequencies. Such range offrequencies may be suitable, for example, for narrow band modulation,frequency modulation, amplitude modulation, or any other suitable typeof modulation.

The primary clock signal is applied to a telemetry phase shiftingcircuit (TX SHIFT) 44 where it may be converted into a phase shiftedclock signal based on a DELAY CONTROL signal and where it may also bemodulated with a data signal, referred to in FIGS. 3 and 4 as the FT MODsignal. TX SHIFT 44 may derive its operating power from the powersource, referred to as VCC, of the BTE unit. TX SHIFT 44 may provide twosignals (which may be referred to as the first and second switchsignals) to network 46 over lines 45 a and 45 b. The application ofthese signals to network 46 may result in a push-pull mode of operation.One signal (e.g., the second switch signal) may be phase shifted withrespect to the other signal (e.g., the first switch signal) based on theDELAY CONTROL signal. These signals, when applied to network 46, producean RF output signal, on signal line 33, including a sinusoidal signalhaving an amplitude that varies as a function of the phase shift betweenthe first and second switch signals. Furthermore, the RF output signalmay be modulated with data in a selected fashion as a function of the FTMOD signal. The RF output signal may then be applied to an antenna coilwithin the headpiece 34, where it is transmitted or coupled as a forwardcarrier signal, through the barrier 18 to a remote receiver (e.g., ICS14).

The power level of the RF output signal may be selected or adjusted, asrequired, to assume various values by controlling the DELAY CONTROLsignal. As explained more fully below, control over the power level ofthe signal provided to the implant device is made more efficient byselecting the phase delay of the second switch signal with respect tothe first switch signal to be “just right”, not too small (which mayresult in improper operation of the implant device), and not too large(which may result in the implant device being overdriven, and would thusrepresent a waste of energy).

Headpiece 34 may also include an antenna coil tuned to receive abacktelemetry signal from the implanted receiver or device. In someembodiments, in order to simplify the design of BTE unit 32, the backtelemetry feature may be omitted. When used, such a backtelemetry signalis modulated with data from the ICS 14, and is typically at a differentcarrier frequency than is the forward carrier signal transmitted to theimplanted receiver. For example, in one embodiment, where the forwardcarrier signal operates at a fixed frequency of 49 MHz, thebacktelemetry signal may have a fixed carrier frequency of 10.7 MHz. Anexample of one type of modulation used to modulate the backtelemetrysignal may be frequency modulation (FM), but other types of modulationcan also be used.

The backtelemetry signal may be routed through a separate network (notshown) and applied to a first bandpass filter circuit (BPF) 53 oversignal line 49. The filtered backtelemetry signal is then directed, oversignal path 51, to an FM receiver circuit (FM RCVR) 48. FM RCVR 48detects and demodulates the signal it receives over signal line 51.Typically, FM RCVR 48 may utilize a second BPF 54 to aid in thedetection and demodulation process. As a result of such demodulation,two signals are generated by FM RCVR 48 and presented to the othercircuits within BTE unit 32. Such two other signals include a datasignal (BT DATA) that represents the demodulated data received throughthe backtelemetry signal, and a signal (TEL SIG) that identifies thepresence of a backtelemetry signal within the FM RCVR 48. The presenceof the TEL SIG signal may thus be used to identify that a link has beenestablished with ICS 14. Knowing that a link has been established withan ICS may, in turn, be used for various purposes (such as a powercontrol feedback loop). Examples of such purposes may be seen, forexample, in U.S. Pat. No. 5,584,869, which is incorporated herein byreference in its entirety.

If desired, an amplifier (not shown) may be used to further control thepower level of the RF output signal. Such an amplifier may receive theRF output signal from network 46 and provide an amplified variant ofthat signal to headpiece 34. The amplification of the RF output signalmay be fixed or variable. In fixed amplification embodiments, control ofthe output signal amplitude may be controlled by the phase shiftingcircuitry. In variable amplification embodiments, control of the outputsignal amplitude may be controlled by one or both the phase shiftingcircuitry and the amplifier.

OSC 42, TX SHIFT 44, and FM RCVR 48 (when used) may all preferably beformed or embedded within the same application specific integratedcircuit (ASIC) 40. Such ASIC 40 may also include the other digitalcircuits associated with BTE unit 32, such as the speech processingcircuits and control circuitry, and hence the ASIC 40 may be referred toas the BTE ASIC. BTE ASIC 40 may be mounted on a suitable pc board (PCB)within the BTE unit 32. Other discrete components, not part of the ASIC40, may then be mounted, as required, on the BTE PCB or otherwise housedwithin the BTE unit. Such other discrete components may include forexample, in addition to the battery (not shown in FIG. 3), a crystal 43used with OSC 42 to precisely control the frequency F1 of the OSC 42,network 46, and BPF 54.

FIG. 4 shows a schematic of an implementation of TX SHIFT 44 and network46 of FIG. 3 that is in accordance with the principles of the presentinvention. As shown in FIG. 4, TX SHIFT 44 includes variable voltagedelay line (VVDL) circuitry 102 (e.g., phase shifting circuitry), firstamplifier 110, second amplifier 112, and logic circuitry 114. VVDLcircuitry 102 may receive a DELAY CONTROL signal, logic circuitry 114may receive the clock signal F1 and the FT MOD signal. Clock signal F1may be a pulse stream operating at a predetermined clocking frequencyand which pulses between a nominal voltage level (e.g., about 0 volts)and a predetermined voltage level (e.g., ±1.75 volts). The pulse mayhave voltage levels suitable for driving first and second amplifiers 110and 112 HIGH and LOW.

As also shown in FIG. 4 is network 46, including transformer 126, whichmay receive the first and second switch signals on lines 45 a and 45 b,respectively and capacitors 122 and 124. Those skilled in the art willappreciate that capacitors 122 and 124 may be series-resonatingcapacitors that enable network 46 to operate as a double-tuned bandpassfilter. Such bandpass filter advantageously eliminates the need to useadditional filters downstream of network 46, thereby promoting energytransfer efficiency of the present invention.

Moreover, capacitors 122 and 124 may be used to counteract inductance oftransformer 126. Additionally, capacitor 122 may prevent DC current frompassing to transformer 126 and filter out harmonics produced by thedriving action of amplifiers 110 and 112. Transformer 126 may be aloosely coupled transformer having a 1-to-N turns ratio, where N is anarbitrary value. Also, transformer 126 isolates amplifiers 110 and 112from the load.

Amplifiers 110 and 112 may be driven by clock signal F1 and delayedclock signal FD, respectively, to provide the first and second switchsignals to network 46. As shown in FIG. 4, amplifiers 110 and 112 areconnected to a power source (e.g., VCC) and to ground. As shown in thisembodiment, a single power supply is used, which reduces the number ofcomponents required. Additionally, the VCC may be the primary battery ofthe BTE or Speech Processor.

Amplifiers 110 and 112 may be constructed to operate as an H-bridge. Assuch, amplifiers 110 and 112 may each include one or more bi-directionalcurrent carrying devices (e.g., a conventional CMOS digital inverter) toprovide a switching operation (e.g., where amplifiers 110 and 112 can beturned ON and OFF) to control the amplitude of the RF output signal. Thepower source, ground, amplifiers 110 and 112, and network 46 may form acircuit loop that sets the power level of the RF output signal accordingto clock signal F1 and delayed clock signal FD.

Those of skill in the art will appreciate that many different availabledevices can carry a bi-directional current as used in an H-bridgedriver. For example, a uni-directional transistor (e.g., a bipolartransistor) that is bypassed with a diode or other circuit element mayprovide a bi-directional current carrying capacity. Other examples ofbi-directional circuitry include insulated gate bipolar transistors andCMOS inverters.

FIG. 4A shows an illustrative, but more detailed, schematic of portionsof TX SHIFT 44 and network 46 of FIG. 4 in which amplifiers 110 and 112are shown as CMOS inverters in accordance with the principles of thepresent invention. Each CMOS inverter may have two transistors, with onebeing coupled to VCC and the other being coupled to ground. These fourtransistors, labeled T1, T2, T3 and T4, may form a driver configuration(e.g., an H-bridge) where network 46, particularly the primary windingof transformer 126, is treated as a “load.” Several different circuitpaths exist depending on the states of F1 and FD. For example, when F1is HIGH and FD is LOW, current from VCC of amplifier 112 may be routedthrough transformer 126 to ground of amplifier 110. When F1 and FD areboth HIGH, the respective ends of the primary winding of transformer arecoupled to ground via T2 and T4. When F1 and FD are both LOW, therespective ends of the primary winding of transformer 126 are bothcoupled to VCC via T1 and T3.

To facilitate the discussion of how the power level of the RF outputsignal is controlled, reference will be made to FIGS. 5, 6, and 7. Asstated above, TX SHIFT 44 provides the first and second switch signalson lines 45 a and 45 b to network 46. An example of the first and secondswitch signals as applied to lines 45 a and 45 b is shown in FIG. 5. Thecombination of the first and second switch signals may at given timeswithin a clock cycle result in the application of a differential signalacross transformer 126 of network 46 (as shown in FIG. 5). Thisdifferential signal may be the result of a voltage difference of thefirst and second switch signals at transformer 126, which sets the powerlevel of the RF output signal.

In accordance with the present invention, a differential voltage mayexist across transformer 126 when amplifiers 110 and 112 are providingsignals in opposite states; that is one amplifier provides a HIGH signaland the other provides a LOW signal. As shown in FIG. 5, when line 45 ais HIGH (e.g., because amplifier 110 is being driven HIGH) and line 45 bis LOW (e.g., because amplifier 112 is being drive LOW), a differentialvoltage (shown as the triangular portion of the waveform) may existacross transformer 126. A differential voltage may not exist acrosstransformer 126 when amplifiers 110 and 112 output signals in the samestate (e.g., both HIGH or LOW). This is shown in FIG. 5 when both lines45 a and 45 b are in the same state. Thus, when both lines 45 a and 45 bare HIGH, the differential voltage is negligible, and when both lines 45a and 45 b are LOW, the differential voltage is also negligible.

The differential signal may contain frequency components at thefundamental frequency of the carrier, and all odd harmonics. Evenharmonics may not be present in the differential signal because of abalanced operation of amplifiers 110 and 112. That is, amplifiers 110and 112 are balanced because they are substantially identical andsymmetrically driven. Moreover, such balance or symmetric driving of theamplifiers may enable addition/subtraction of the first and secondswitch signals to occur in network 46.

The power level of the carrier signal is controlled by adjusting thephase shift between the second switch signal (on line 45 b) and thefirst switch signal (on line 45 a). Referring back to FIG. 4, TX SHIFT44 may control the level of the differential voltage across transformer126 (and consequently the amplitude of the RF output signal) byselectively phase shifting clock signal F1. VVDL 102 may phase shiftclock signal F1 based on the DELAY CONTROL signal to produce delayedclock signal FD, which drives amplifier 112. The extent to which clocksignal F1 is delayed may be proportional to the voltage level of theDELAY CONTROL signal applied to VVDL 102. Thus, depending on the voltagelevel of DELAY CONTROL signal, the delayed clock signal FD may be phaseshifted anywhere from −180° to 0° to 180° with respect to clock signalF1.

To better explain how phase shifting is implemented in accordance withthe principles of the present invention to control the amplitude of theRF output signal, reference is made to FIGS. 6 and 7. FIG. 6 showsseveral timing waveform diagrams in accordance with the principles ofthe present invention. FIG. 7 shows a graph of the amplitude of the RFoutput signal as a function of phase shift in accordance with theprinciples of the present invention.

Referring now to FIG. 6, the top waveform represents the clock signalF1, having a cycle period of T1. Note that the period T1 is the inverseof the clock signal F1 (i.e., T1=1/F1). The F1 waveform may be thewaveform that drives amplifier 110. The waveforms below the top waveform(the F1 waveform) represent waveforms of phase shifted clock signals andmay represent a waveform that drives amplifier 112. These phase shiftedwaveforms are denoted as FD_(N), where N is an angular value ranging−180° to 180°. Note that radian values ranging −π to π may be used, as πis equivalent to an angular value of 180°. Further note that onlywaveforms corresponding to positive angular values are shown in FIG. 6to avoid overcrowding the FIG., even though both positive and negativeangular values are shown in FIG. 7. FIG. 6 shows that the phase shift ofthe various waveforms is denoted as ΦN, where N represents an angularvalue.

The FD_(0°) waveform represents a time delayed clock signal having aphase delay of 0°. As shown in FIG. 6, the leading edge of FD_(0°) issubstantially co-linear with the leading edge of F1. Thus, FD_(0°) issubstantially the same as F1 at Φ_(0°). Moreover, when the phase delayis 0°, the differential voltage across transformer may be negligible orabout 0 volts, resulting in negligible power being carried by the RFoutput signal, as shown in FIG. 7. Note that when the phase delay is 0°,both amplifiers 110 and 112 provide signals in the same state (e.g.,both HIGH or LOW) for the entire duration of the clock period.

The waveform FD_(180°) represents a phase shifted clock signal having aphase delay of 180°. As shown in FIG. 6, the leading edge of FD_(180°)is delayed by Φ_(180°) with respect to the leading edge of F1. Such adelay may result in a maximum power level (or amplitude) for the RFoutput signal (shown in FIG. 7) because the average differential voltageacross transformer 126 is maximized for a given clock period. Theaverage voltage across transformer 126 may be maximized because theoutput of amplifiers 110 and 112 are in opposite states for the entireduration of the clock period.

The FD_(0°) and the FD_(180°) waveforms may represent the extreme endsof phase shifting in accordance with the invention, and thus correspondto the minimum and maximum power levels of the RF output signal. Theother waveforms (e.g., FD_(45°) and FD_(90°)) represent time delayedclock signals having phase delays ranging between 0° and 180°. It willbe understood that by varying the phase delay to a value between 0° and180° practically any desired power level (or amplitude) can be obtainedfor the RF output signal.

Several advantages of using TX SHIFT 44 are realized over the prior art.One advantage is that the maximum magnitude of power transmitted to theload is greater than the maximum magnitude of power that can bedelivered by, for example, a single-ended network, with all otherrelevant factors (e.g., battery voltage level) being equal. For example,the push-pull mode of operation of the present invention can deliver upto 6 db more power than a single-ended network given the same loadimpedance and power supply, VCC. Moreover, prior-art single-endednetworks require voltage regulator circuitry (e.g., a variable voltageregulator or switching regulator) with discrete components that must beadded external to the integrated circuit containing the transmitterdigital circuits, whereas the present invention eliminates the need forsuch additional discrete components.

An advantage realized in a push-pull mode of operation of operation isthat amplifiers 110 and 112 are balanced. That is, the driving action ofamplifiers 110 and 112 may result in the production of odd harmonics ofthe carrier signal, but not any even harmonics. Thus, the powerefficiency is enhanced because network 46 need only filter out thehigher order odd harmonics and not all higher order harmonics.

Control circuitry (not shown) may provide the DELAY CONTROL signal andFT MOD signal to TX SHIFT 44 and may receive the BT DATA and TEL SIGsignals (as discussed above in connection with FIG. 3). The controlcircuitry may provide a DELAY CONTROL signal of an appropriate level(e.g., voltage level) to control the phase shift of FD, therebycontrolling the magnitude of power transmitted on the RF output signalto the implanted ICS 14. The level of the DELAY CONTROL signal may bebased on, for example, the received TEL SIG or BT DATA signal. Moreover,the level of the DELAY CONTROL signal may be such that just the rightquantity of power is supplied to the implanted ICS 14.

The FT MOD signal, which may contain data representing audio signalsreceived by BTE unit 32, may be incorporated into the carrier signalbeing transmitted to the implanted ICS 14. FIG. 8 depicts one manner inwhich the carrier clock signal F1 may be modulated with digital datausing ON/OFF keying. For example, if FT MOD and F1 are fed to logiccircuitry 116 (e.g., an AND gate), the data in the FT MOD data streamcan be used to modulate the carrier signal, F1. In accordance with sucha modulation scheme, a stream of bit periods, of mT1 seconds each, areprovided in which the carrier signal data pulses are present or notpresent, where m is the number of data pulses included in each bitperiod, and the time interval between successive data pulses is T1seconds. Thus, for example, if T1 is equal to 20 nanoseconds, and m isequal to 10, then each bit period is 10*20=200 nanoseconds, or 0.20 μsec(where “μsec” stands for microseconds). The data scheme shown in FIG. 8assumes that a digital “1” is represented by the presence of a burst ofpulses during the data bit time, and that a digital “0” is representedby the absence of a burst of pulses during the data bit time. (Thisassignment, of course, could just as easily be reversed, with a digital“0” being represented by the presence of the data pulses, and a digital“1” being represented by the absence of the data pulses.) Thus, the datastream shown in FIG. 8 is representative of a digital data sequence:

“10101101101 . . . .”

Such a digital data sequence may, in turn, be encoded using a suitableencoding scheme, as is known in the art, to create a sequence of digitaldata words at a desired Baud rate.

It will be understood that the present invention is not limited to theforegoing technique for incorporating data into the carrier signal. Forexample, other techniques such as frequency modulation can be used.

While the application of the transmitter circuitry of the presentinvention is described primarily in the context of a cochlearstimulation system (e.g., an BTE cochlear stimulation system), thosewith skill in the art will appreciate that the present invention may beapplied to any type of system that requires generation of high frequencyRF signals that are transmitted at minimal power consumption.

Thus it is seen that the present invention provides a compact, lowpower, highly efficient, RF telemetry transmitter circuit that may beused to transfer RF power from a limited power source (e.g., a smallbattery) through a barrier, such as the skin, to a device on the otherside of the barrier (e.g., an implant device). It is further seen thatsuch invention finds particular applicability for use within abehind-the-ear unit of a cochlear implant system. It is also seen thatthe invention provides a way to control the drive level of a fixedfrequency RF carrier signal by phase shifting a fixed frequency carrierinput signal. A person skilled in the art will appreciate that thepresent invention can be practiced by other than the describedembodiments, which are presented for purposes of illustration ratherthan of limitation, and the present invention is limited only by theclaims which follow.

1. A radio frequency (RF) telemetry transmitter for use in a cochlearimplant system, the RF telemetry transmitter comprising: a clock sourceconfigured to provide a clock signal; a phase shifting circuit coupledto said clock source and configured to receive a delay control signaland generate a phase shifted clock signal which is phase shiftedrelative to said clock signal by a predetermined phase delay that is setby the delay control signal; switching circuitry configured to receivesaid clock signal and said phase shifted clock signal and output a firstswitch signal and a second switch signal, said second switch signalbeing phase shifted relative to said first switch signal by saidpredetermined phase delay; and a network coupled to said switchingcircuitry and configured to generate an output signal to be transmittedto an implantable receiver circuit within an implant part of saidcochlear implant system, said output signal containing power and data inaccordance with said first and second switch signals; wherein saidnetwork is configured to adjust a power level of said output signal inresponse to an adjustment in said predetermined phase delay.
 2. The RFtelemetry transmitter of claim 1, wherein said predetermined phase delayranges from −180° to 180°.
 3. The RF telemetry transmitter of claim 1,further comprising: control circuitry coupled to said clock source andsaid phase shifting circuit, said control circuitry operative to:provide the delay control signal to said phase shifting circuit; andprovide a modulation signal comprising said data to be transmitted tosaid implant part, wherein said modulation signal modulates said clocksignal to incorporate said data into said output signal.
 4. The RFtelemetry transmitter of claim 1, wherein said network is a push-pullnetwork comprising a transformer, and wherein said switching circuitrycomprises: a first switch responsive to said clock signal to apply saidfirst switch signal to a first node of said transformer; and a secondswitch responsive to said phase shifted clock signal to apply saidsecond switch signal to a second node of said transformer, wherein theapplication of said first and second switch signals is configured tocause a differential voltage to exist across said transformer.
 5. The RFtelemetry transmitter of claim 1, further comprising: an antenna coilcoupled to said network and configured to transmit said output signal tosaid implant part.
 6. The RF telemetry transmitter of claim 1, whereinsaid clock signal comprises a variable frequency signal.
 7. The RFtelemetry transmitter of claim 1, wherein said clock signal comprises afixed frequency signal.
 8. The RF telemetry transmitter of claim 1,wherein said implant part is configured to generate said stimulationcurrent in accordance with said data contained within said outputsignal.
 9. The RF telemetry transmitter of claim 1, wherein said networkis configured to increase a power level of said output signal inresponse to an increase in said predetermined phase delay.
 10. The RFtelemetry transmitter of claim 1, wherein said network is configured todecrease a power level of said output signal in response to a decreasein said predetermined phase delay.
 11. A cochlear implant systemcomprising: an external part having a radio frequency (RF) telemetrytransmitter; and an implant part having an implantable receiver circuitfor generating and applying a stimulation current to a selected pair ofimplantable electrodes; wherein said external part comprises a clocksource configured to provide a clock signal, and a phase shiftingcircuit coupled to said clock source and configured to generate saidphase shifted clock that is set by a delay control signal received bythe phase shifting circuit, switching circuitry configured to receivethe clock signal and the phase shifted clock signal and output a firstswitch signal and a second switch signal, said second switch signalbeing phase shifted relative to said first switch signal by saidpredetermined phase delay, a network coupled to said switching circuitryand configured to generate a carrier signal in accordance with saidfirst and second switch signals, and an antenna coil configured totransmit said carrier signal to said implantable receiver circuit;wherein said network is configured to adjust a power level of saidcarrier signal in response to an adjustment in said predetermined phasedelay.
 12. The system of claim 11, wherein said network is configured toincrease a power level of said output signal in response to an increasein said predetermined phase delay.
 13. The system of claim 11, whereinsaid network is configured to decrease a power level of said outputsignal in response to a decrease in said predetermined phase delay. 14.The system of claim 11, wherein said implant part is configured togenerate said stimulation current in accordance with data containedwithin said carrier signal.
 15. A radio frequency (RF) telemetrytransmitter for use in a cochlear implant system, the RF telemetrytransmitter comprising: switching circuitry configured to receive aclock signal and a phase shifted clock signal and output a first switchsignal and a second switch signal, said second switch signal being phaseshifted relative to said first switch signal by a predetermined phasedelay that is set by a delay control signal; a network coupled to saidswitching circuitry and configured to generate a carrier signal inaccordance with said first and second switch signals, and an antennacoil configured to transmit said carrier signal to said implantablereceiver circuit; wherein said network is configured to adjust a powerlevel of said carrier signal in response to an adjustment in saidpredetermined phase delay.
 16. The RF telemetry transmitter of claim 15,wherein said external part further comprises: a clock source configuredto provide said clock signal; and a phase shifting circuit coupled tosaid clock source and configured to generate said phase shifted clock.17. The RF telemetry transmitter of claim 15, wherein said network isconfigured to increase a power level of said output signal in responseto an increase in said predetermined phase delay.
 18. The RF telemetrytransmitter of claim 15, wherein said network is configured to decreasea power level of said output signal in response to a decrease in saidpredetermined phase delay.